New Insight into Procedure of Interface Electron Transfer through Cascade System with Enhanced Photocatalytic Activity

Recombination of photogenerated electron–hole pairs is extremely limited in the practical application of photocatalysis toward solving the energy crisis and environmental pollution. A rational design of the cascade system (i.e., rGO/Bi₂WO₆/Au, and ternary composites) with highly efficient charge carrier separation is successfully constructed. As expected, the integrated system (rGO/Bi₂WO₆/Au) shows enhanced photocatalytic activity compared to bare Bi₂WO₆ and other binary composites, and it is proved in multiple electron transfer (MET) behavior, namely a cooperative electron transfer (ET) cascade effect. Simultaneously, UV–vis/scanning electrochemical microscopy is used to directly identify MET kinetic information through an in situ probe scanning technique, where the “fast” and “slow” heterogeneous ET rate constants (K_eff) of corresponding photocatalysts on the different interfaces are found, which further reveals that the MET behavior is the prime source for enhanced photocatalytic activity. This work not only offers a new insight to study catalytic performance during photocatalysis and electrocatalysis systems, but also opens up a new avenue to design highly efficient catalysts in photocatalytic CO₂ conversion to useful chemicals and photovoltaic devices.     

Polypyrrole/Bi2WO6 composite with high charge separation efficiency and enhanced photocatalytic activity

The recombination of photogenerated electrons and holes is a crucial factor that limits the efficiency of photocatalysis and dye-sensitized solar cells. Conducting polymers are known to have high charge carrier mobility. Herein, a polypyrrole (PPy)/Bi₂WO₆ composite with promoted charge separation efficiency was designed by a “photocatalytic oxidative polymerization” method. The photo-degradation of a typical model pollutant, phenol, demonstrated that the PPy/Bi₂WO₆ composite possessed significantly enhanced photo-activity than pure Bi₂WO₆ under simulated sunlight irradiation. The higher photo-activity was attributed to the synergetic effect between PPy and Bi₂WO₆. The photogenerated holes on the valence band of Bi₂WO₆ could transfer to the highest occupied molecular orbital of PPy, leading to rapid photoinduced charge separation and enhancing the photocatalytic activity. This work provided a new concept for rational design and development of highly efficient polymer-semiconductor photocatalysts for environmental purification under simulated sunlight.     

Fabrication of plate-on-plate Z-scheme SnS<Subscript>2</Subscript>/Bi<Subscript>2</Subscript>MoO<Subscript>6</Subscript> heterojunction photocatalysts with enhanced photocatalytic activity

A class of direct plate-on-plate Z-scheme heterojunction SnS₂/Bi₂MoO₆ photocatalysts was synthesized via a two-step hydrothermal method. The materials were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Fourier transform infrared photoluminescence emission spectra, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity was estimated via the degradation of crystal violet (CV) and ciprofloxacin (CIP). The experimental results indicated that the 5 wt% SnS₂/Bi₂MoO₆ composites exhibited significantly enhanced performance in contrast to pure Bi₂MoO₆ or SnS₂ nanoflakes, and were also superior to the popular TiO₂ (P25). The degradation reaction accorded well with the first-order reaction kinetics equation; the rate constant of CV using a SnS₂ content of 5 wt% photocatalyst was ~?3.6 times that of the Bi₂MoO₆ and 2.4 times that of SnS₂. Furthermore, a SnS₂ content of 5 wt% exhibited a 1.7 times higher photocatalytic activity of CIP than that of pure Bi₂MoO₆, and 1.3 times that of pure SnS₂. Radical trapping experiments and an electron spin resonance technique indicated that h⁺ and ·OH were the dominant active species involved in the degradation process. A plasmonic Z-scheme photocatalytic mechanism was proposed to explain the superior photocatalytic activities and efficient separation of photogenerated electrons and holes.     

Defect‐Tailoring Mediated Electron–Hole Separation in Single‐Unit‐Cell Bi3O4Br Nanosheets for Boosting Photocatalytic Hydrogen Evolution and Nitrogen Fixation

Solar photocatalysis is a potential solution to satisfying energy demand and its resulting environmental impact. However, the low electron–hole separation efficiency in semiconductors has slowed the development of this technology. The effect of defects on electron–hole separation is not always clear. A model atomically thin structure of single‐unit‐cell Bi₃O₄Br nanosheets with surface defects is proposed to boost photocatalytic efficiency by simultaneously promoting bulk‐ and surface‐charge separation. Defect‐rich single‐unit‐cell Bi₃O₄Br displays 4.9 and 30.9 times enhanced photocatalytic hydrogen evolution and nitrogen fixation activity, respectively, than bulk Bi₃O₄Br. After the preparation of single‐unit‐cell structure, the bismuth defects are controlled to tune the oxygen defects. Benefiting from the unique single‐unit‐cell architecture and defects, the local atomic arrangement and electronic structure are tuned so as to greatly increase the charge separation efficiency and subsequently boost photocatalytic activity. This strategy provides an accessible pathway for next‐generation photocatalysts.     

Graphene in Photocatalysis: A Review

In recent years, heterogeneous photocatalysis has received much research interest because of its powerful potential applications in tackling many important energy and environmental challenges at a global level in an economically sustainable manner. Due to their unique optical, electrical, and physicochemical properties, various 2D graphene nanosheets‐supported semiconductor composite photocatalysts have been widely constructed and applied in different photocatalytic fields. In this review, fundamental mechanisms of heterogeneous photocatalysis, including thermodynamic and kinetics requirements, are first systematically summarized. Then, the photocatalysis‐related properties of graphene and its derivatives, and design rules and synthesis methods of graphene‐based composites are highlighted. Importantly, different design strategies, including doping and sensitization of semiconductors by graphene, improving electrical conductivity of graphene, increasing eloectrocatalytic active sites on graphene, strengthening interface coupling between semiconductors and graphene, fabricating micro/nano architectures, constructing multi‐junction nanocomposites, enhancing photostability of semiconductors, and utilizing the synergistic effect of various modification strategies, are thoroughly summarized. The important applications including photocatalytic pollutant degradation, H₂ production, and CO₂ reduction are also addressed. Through reviewing the significant advances on this topic, it may provide new opportunities for designing highly efficient 2D graphene‐based photocatalysts for various applications in photocatalysis and other fields, such as solar cells, thermal catalysis, separation, and purification.     

Preparation of a 3D flower-like spherical structure g-C3N4/CuBi2O4/Bi2MoO6 photocatalyst for efficient removal of antibiotics under visible light

For the remediation of antibiotic-contaminated water bodies, this study synthesized g-C₃N₄/CuBi₂O₄/Bi₂MoO₆ 3D flower-like spherical photocatalysts by a solvothermal method. The tetracycline antibiotics were used as the target pollutants and degraded under visible light to evaluate the photocatalytic performance of the prepared photocatalysts. Notably, the g-C₃N₄/CuBi₂O₄/Bi₂MoO₆ photocatalyst achieved 84.6 % and 91.6 % for the degradation of tetracycline hydrochloride and chlortetracycline (100 mL, 20 mg/L), respectively, within 2 h under visible light irradiation. Furthermore, we found that the composites showed very low degradation rates for dye-based contaminants, but still exhibited excellent photocatalytic activity for antibiotics in a mixed contaminant system of dyes and antibiotics. And the intermediate was detected by gas chromatography-mass spectrometry (GC–MS), suggesting a possible photo-degradation pathway for tetracycline. Finally, biochemical experiments were carried out to further illustrate the effective degradation of antibiotics in water after photocatalytic degradation by observing and comparing the growth of mung bean seeds.     

Hybridized 2D Nanomaterials Toward Highly Efficient Photocatalysis for Degrading Pollutants: Current Status and Future Perspectives

Organic pollutants including industrial dyes and chemicals and agricultural waste have become a major environmental issue in recent years. As an alternative to simple adsorption, photocatalytic decontamination is an efficient and energy‐saving technology to eliminate these pollutants from water environment, utilizing the energy of external light, and unique function of photocatalysts. Having a large specific surface area, numerous active sites, and varied band structures, 2D nanosheets have exhibited promising applications as an efficient photocatalyst for degrading organic pollutants, particularly hybridization with other functional components. The novel hybridization of 2D nanomaterials with various functional species is summarized systematically with emphasis on their enhanced photocatalytic activities and outstanding performances in environmental remediation. First, the mechanism of photocatalytic degradation is given for discussing the advantages/shortcomings of regular 2D materials and identifying the importance of constructing hybrid 2D photocatalysts. An overview of several types of intensively investigated 2D nanomaterials (i.e., graphene, g‐C₃N₄, MoS₂, WO₃, Bi₂O₃, and BiOX) is then given to indicate their hybridized methodologies, synergistic effect, and improved applications in decontamination of organic dyes and other pollutants. Finally, future research directions are rationally suggested based on the current challenges.     

One-pot solvothermal synthesis of heterogeneous Bi/Bi2WO6/BiOBr composite with high adsorption and photocatalytic performance

Adsorption and photocatalysis are two effective strategies to govern the worsening water pollution problem. Exploitation of multifunctional adsorbents and/or photocatalysts is one of the most attractive frontiers of current research. Bismuth-based compounds have stood out from enormous candidates in virtue of their abundance, low toxicity and high efficiency. In this study, heterogeneous Bi/Bi₂WO₆/BiOBr composite is synthesized via a facile one-pot solvothermal method. Benefiting from the large specific surface area, enhanced light absorption and high separation efficiency of the photogenerated electrons and holes, the tri-phase Bi/Bi₂WO₆/BiOBr composite exhibits outstanding adsorption performance for the cationic dye Rhodamine B and superior photocatalytic performance for Methyl Orange. The Bi/Bi₂WO₆/BiOBr composite degrades?~?97.8% of MO within 60 min, whose reaction rate is 13 times of Bi₂WO₆, 5 times of Bi/Bi₂WO₆, 1.9 times of Bi/BiOBr and 4.8 times of BiOBr. Simultaneously, the Bi/Bi₂WO₆/BiOBr photocatalyst shows excellent recycling efficiency. The multifunctional heterostructural composite holds promise for applications in the field of environmental treatment.

     

Mechanochemical homodisperse of Bi2MoO6 on Zn-Al LDH matrix to form Z-scheme heterojunction with promoted visible-light photocatalytic performance

In this work, a Z-scheme Bi₂MoO₆/Zn-Al LDH heterojunction photocatalyst with excellent visible light responsiveness was fabricated via a two-step mechanochemical ball-milling process, where amorphous Bi₂MoO₆ particles were homodispersed upon the surface of lamellar LDH matrix. BPA was selected as the targeted organic contaminant to quantitatively evaluate the photocatalytic capacity of Bi₂MoO₆/Zn-Al LDH, in which the optimal 30 wt%Bi₂MoO₆/LDH exhibited a degradation rate of 96% within 300 min, over 9.25 and 18.5 times higher than that of individual pristine Bi₂MoO₆ and LDH, respectively. The crystal structure, microtopography, interfacial physicochemical interaction, optical and electrochemical properties of as-fabricated hybrids were systematically evaluated, and the DFT theoretical calculation was used to confirm the electronic structural characteristics in the Bi₂MoO₆/LDH heterojunction. A possible photocatalysis reaction mechanism was interpreted through ESR where the major manner of ?O^2– proved the Z-scheme electron migration within matched band levels.     

Ultrathin 2D Photocatalysts: Electronic‐Structure Tailoring,Hybridization, and Applications

As a sustainable technology, semiconductor photocatalysis has attracted considerable interest in the past several decades owing to the potential to relieve or resolve energy and environmental‐pollution issues. By virtue of their unique structural and electronic properties, emerging ultrathin 2D materials with appropriate band structure show enormous potential to achieve efficient photocatalytic performance. Here, the state‐of‐the‐art progress on ultrathin 2D photocatalysts is reviewed and a critical appraisal of the classification, controllable synthesis, and formation mechanism of ultrathin 2D photocatalysts is presented. Then, different strategies to tailor the electronic structure of ultrathin 2D photocatalysts are summarized, including component tuning, thickness tuning, doping, and defect engineering. Hybridization with the introduction of a foreign component and maintaining the ultrathin 2D structure is presented to further boost the photocatalytic performance, such as quantum dots/2D materials, single atoms/2D materials, molecular/2D materials, and 2D–2D stacking materials. More importantly, the advancement of versatile photocatalytic applications of ultrathin 2D photocatalysts in the fields of water oxidation, hydrogen evolution, CO₂ reduction, nitrogen fixation, organic syntheses, and removal pollutants is discussed. Finally, the future opportunities and challenges regarding ultrathin 2D photocatalysts to bring about new opportunities for future research in the field of photocatalysis are also presented.     

Polymeric Photocatalysts Based on Graphitic Carbon Nitride

Semiconductor‐based photocatalysis is considered to be an attractive way for solving the worldwide energy shortage and environmental pollution issues. Since the pioneering work in 2009 on graphitic carbon nitride (g‐C₃N₄) for visible‐light photocatalytic water splitting, g‐C₃N₄‐based photocatalysis has become a very hot research topic. This review summarizes the recent progress regarding the design and preparation of g‐C₃N₄‐based photocatalysts, including the fabrication and nanostructure design of pristine g‐C₃N₄, bandgap engineering through atomic‐level doping and molecular‐level modification, and the preparation of g‐C₃N₄‐based semiconductor composites. Also, the photo­catalytic applications of g‐C₃N₄‐based photocatalysts in the fields of water splitting, CO₂ reduction, pollutant degradation, organic syntheses, and bacterial disinfection are reviewed, with emphasis on photocatalysis promoted by carbon materials, non‐noble‐metal cocatalysts, and Z‐scheme heterojunctions. Finally, the concluding remarks are presented and some perspectives regarding the future development of g‐C₃N₄‐based photocatalysts are highlighted.     

Multiple heterojunction system of Bi2MoO6/WO3/Ag3PO4 with enhanced visible-light photocatalytic performance towards dye degradation

Multiple heterojunction system of Bi₂MoO₆/WO₃/Ag₃PO₄ was designed via constructing binary heterojunction Bi₂MoO₆/WO₃, followed by the deposition of nano-Ag₃PO₄ on the surface of Bi₂MoO₆/WO₃. Various techniques were employed to characterize the properties of the as-prepared catalytic system. In this study, the decomposition efficiency of C.I. reactive blue 19 (RB-19) was used as a measure of photocatalytic activity and the Bi₂MoO₆/WO₃/Ag₃PO₄ composite exceeded its stand-alone components (pristine Ag₃PO₄, WO₃/Ag₃PO₄ and Bi₂MoO₆/Ag₃PO₄) by 3.16 times, 2.63 times and 1.75 times, respectively. The photocatalytic tests implied that the construction of multiple heterojunction could achieve efficient separation of photo-generated electrons and holes. A possible photocatalytic mechanism for Bi₂MoO₆/WO₃/Ag₃PO₄ system was also proposed according to the results of trapping experiments.     

Novel Mo/Bi2MoO6/Bi3ClO4 heterojunction photocatalyst for ultra-deep desulfurization of thiophene under simulated sunlight irradiation

In the present work, a visible-light-driven Mo/Bi₂MoO₆/Bi₃ClO₄ heterojunction photocatalyst was fabricated via the Pechini sol–gel process. The type and amount of gelling agent, chelating agent and mole ratio of chelating agent to total metals were balanced to generate ultrafine nanoparticles. The Mo/Bi₂MoO₆/Bi₃ClO₄ nanocomposite as a novel photocatalyst not only exhibited an excellent visible-light photocatalytic desulfurization performance of thiophene (~97%), but also had better photodesulfurization efficiency than Mo/Bi₂MoO₆ and Bi₃ClO₄ nanostructures. The ultra-deep photocatalytic desulfurization performance of the Mo/Bi₂MoO₆/Bi₃ClO₄ nanocomposite can be attributed to the strong visible-light absorption, unique nanostructures, high separation and low recombination of electron–hole pairs due to the as-formed heterojunctions. Furthermore, a photocatalytic desulfurization mechanism was elucidated via radical trapping experiments, which revealed that the ^?O₂^? and ^?OH radicals play a key role in the photocatalytic desulfurization process.     

Smart Utilization of Carbon Dots in Semiconductor Photocatalysis

Efficient capture of solar energy will be critical to meeting the energy needs of the future. Semiconductor photocatalysis is expected to make an important contribution in this regard, delivering both energy carriers (especially H₂) and valuable chemical feedstocks under direct sunlight. Over the past few years, carbon dots (CDs) have emerged as a promising new class of metal‐free photocatalyst, displaying semiconductor‐like photoelectric properties and showing excellent performance in a wide variety of photoelectrochemical and photocatalytic applications owing to their ease of synthesis, unique structure, adjustable composition, ease of surface functionalization, outstanding electron‐transfer efficiency and tunable light‐harvesting range (from deep UV to the near‐infrared). Here, recent advances in the rational design of CDs‐based photocatalysts are highlighted and their applications in photocatalytic environmental remediation, water splitting into hydrogen, CO₂ reduction, and organic synthesis are discussed.     

Recent Progress of Single-Atom Photocatalysts Applied in Energy Conversion and Environmental Protection

Photocatalysis driven by solar energy is a feasible strategy to alleviate energy crises and environmental problems. In recent years, significant progress has been made in developing advanced photocatalysts for efficient solar-to-chemical energy conversion. Single-atom catalysts have the advantages of highly dispersed active sites, maximum atomic utilization, unique coordination environment, and electronic structure, which have become a research hotspot in heterogeneous photocatalysis. This paper introduces the potential supports, preparation, and characterization methods of single-atom photocatalysts in detail. Subsequently, the fascinating effects of single-atom photocatalysts on three critical steps of photocatalysis (the absorption of incident light to produce electron-hole pairs, carrier separation and migration, and interface reactions) are analyzed. At the same time, the applications of single-atom photocatalysts in energy conversion and environmental protection (CO₂ reduction, water splitting, N₂ fixation, organic macromolecule reforming, air pollutant removal, and water pollutant degradation) are systematically summarized. Finally, the opportunities and challenges of single-atom catalysts in heterogeneous photocatalysis are discussed. It is hoped that this work can provide insights into the design, synthesis, and application of single-atom photocatalysts and promote the development of high-performance photocatalytic systems.     

Facile preparation of heterostructured Bi2O3/Bi2MoO6 hollow microspheres with enhanced visible-light-driven photocatalytic and antimicrobial activity

Hierarchical β-Bi₂O₃/Bi₂MoO₆ heterostructured flower-like microspheres assembled from nanoplates with different β-Bi₂O₃ loadings (0–26.5 mol%) were synthesized through a one-step template-free solvothermal route. Under visible-light illumination (λ > 420 nm), over 99% of rhodamine B was degraded within 90 min on the 21.9 mol% of β-Bi₂O₃ loading Bi₂O₃/Bi₂MoO₆ microspheres. The remarkable enhancement of photocatalytic activity of the hierarchical Bi₂O₃/Bi₂MoO₆ micro/nanostructures can be attributed to the effective separation of the photoinduced charge carriers at the interfaces and in the semiconductors. The electrons (e⁻) are the main active species in aqueous solution under visible-light irradiation. The Bi₂O₃/Bi₂MoO₆ also displays visible-light photocatalytic activity for the destruction of E. coli. In addition, the β-Bi₂O₃ in the hierarchical Bi₂O₃/Bi₂MoO₆ microspheres is very stable and the composite can be easily recycled by a simple filtration step, thus the second pollution can be effectively avoided. A possible photocatalytic mechanism was proposed based on the experimental results.     

Design of Heterostructured Hollow Photocatalysts for Solar‐to‐Chemical Energy Conversion

Direct conversion of solar energy into chemical energy in a sustainable manner is one of the most promising solutions to the energy crisis and environmental issues. Fabrication of highly active photocatalysts is of great significance for the practical applications of efficient solar‐to‐chemical energy conversion systems. Among various photocatalytic materials, semiconductor‐based heterostructured photocatalysts with hollow features show distinct advantages. Recent research efforts on rational design of heterostructured hollow photocatalysts toward photocatalytic water splitting and CO₂ reduction are presented. First, both single‐shelled and multishelled heterostructured photocatalysts are surveyed. Then, heterostructured hollow photocatalysts with tube‐like and frame‐like morphologies are discussed. It is intended that further innovative works on the material design of high‐performance photocatalysts for solar energy utilization can be inspired.     

Ultrasmall MoOx Clusters as a Novel Cocatalyst for Photocatalytic Hydrogen Evolution

To enhance the performance of semiconductor photocatalysts, cocatalysts are used to accelerate surface reactions. Herein, ultrasmall molybdenum–oxygen (MoO_x) clusters are developed as a novel non‐noble cocatalyst, which significantly promotes the photocatalytic hydrogen generation rate of CdS nanowires (NWs). As indicated by extended X‐ray absorption fine structure analysis, direct bonds are formed between CdS NWs and MoO_x clusters, which guarantee the migration of photo‐generated charge carriers. Moreover, the MoO_x clusters induce deep electron trap states owing to the unique atomic arrangement and configuration with the generation of long‐lived electrons to enhance the activity. These findings may guide the design of efficient cocatalytic materials for solar water splitting and open a new avenue toward practical applications of ultrasmall clusters.     

Facile integration of brown TiO2−x with Bi4V2O11 and BiVO4: Double S-scheme mechanism for exceptional visible-light photocatalytic performance in degradation of pollutants

Heterogeneous photocatalysis has been denoted as a promising approach to dealing with environmental and energy crises. Herein, quantum dots (QDs) of TiO_2?x/Bi₄V₂O₁₁/BiVO₄ (T_OVBBV) heterojunction photocatalysts were successfully synthesized through a facile one-pot hydrothermal process with varying amounts of integrated Bi₄V₂O₁₁ and BiVO₄ semiconductors. The resultant photocatalysts were characterized by XPS, FT-IR, EDX, XRD, PL, EIS, SEM, TEM, HRTEM, BET, and UV–vis DRS techniques, and their photocatalytic efficiencies were examined via removing tetracycline (TC), azithromycin (AZi), and rhodamine B (RhB) under visible-light illumination. The results showed exceptionally promoted photocatalytic degradation efficiencies of the studied pollutants using the ternary T_OVBBV-2 nanocomposite. The T_OVBBV-2 nanocomposite was 45.4, 11.6, and 19.8-times more effective than TiO₂, 7.24, 5.85 and 2.34-folds better than TiO_2?x, and 9.81, 9.23, and 1.61-times more effective than Bi₄V₂O₁₁/BiVO₄ for photooxidation of TC, AZi, and RhB, respectively. The significant enhancement in the photocatalytic efficacy of the T_OVBBV-2 nanocomposite originated from the defective oxygen sites in the titanium dioxide structure, which subsequently facilitated the transfer of photo-induced charge carriers through the formed tandem n-n heterojunctions amongst TiO_2?x, Bi₄V₂O₁₁, and BiVO₄ components. In addition to high photocatalytic activities, the T_OVBBV-2 photocatalyst demonstrated good photostability and durability after concurrent applications. This work recommends the T_OVBBV photocatalyst for facile photocatalytic treatment of pollutants owing to its simple preparation route, high degradation outcomes, and robust structure for practical applications.     

Heterostructured Fe3O4/Bi2O2CO3 photocatalyst: Synthesis,characterization and application in recyclable photodegradation of organic dyes under visible light irradiation

Heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by a two-step method. First, Fe₃O₄ nanoparticles with the size of ca. 10 nm were synthesized by chemical method at room temperature and then heterostructured Fe₃O₄/Bi₂O₂CO₃ photocatalyst was synthesized by hydrothermal method at 180 °C for 24 h with the addition of 10 wt% Fe₃O₄ nanoparticles into the precursor suspension of Bi₂O₂CO₃. The pH value of synthesis suspension was adjusted to 4 and 6 with the addition of 2 M NaOH aqueous solution. By controlling the pH of synthesis suspension at 4 and 6, sphere- and flower-like Fe₃O₄/Bi₂O₂CO₃ photocatalysts were obtained, respectively. Both photocatalysts demonstrate superparamagnetic behavior at room temperature. The UV–vis diffuse reflectance spectra of the photocatalysts confirm that all the heterostructured photocatalysts are responsive to visible light. The photocatalytic activity of the heterostructured photocatalysts was evaluated for the degradation of methylene blue (MB) and methyl orange (MO) in aqueous solution over the photocatalysts under visible light irradiation. The heterostructured photocatalysts prepared in this study exhibit highly efficient visible-light-driven photocatalytic activity for the degradation of MB and MO, and they can be easily recovered by applying an external magnetic field.